Closing in on the Border Between Primordial Plasma and Ordinary Matter

UPTON, NY - Scientists taking advantage of the versatility and new
capabilities of the Relativistic Heavy Ion Collider (RHIC), an atom
smasher at the U.S. Department of Energy's Brookhaven National Laboratory,
have observed first glimpses of a possible boundary separating ordinary
nuclear matter, composed of protons and neutrons, from the seething soup
of their constituent quarks and gluons that permeated the early universe
some 14 billion years ago. Though RHIC physicists have been creating and
studying this primordial quark-gluon plasma (QGP) for some time, the
latest preliminary data-presented at the Quark Matter 2012 international
conference -come from systematic studies varying the energy and types of
colliding ions to create this new form of matter under a broad range of
initial conditions, allowing the experimenters to unravel its intriguing
properties.

"2012 has been a banner year for RHIC, with record-breaking collision
rates, first collisions of uranium ions, and first asymmetric collisions
of gold ions with copper ions," said Samuel Aronson, Director of
Brookhaven National Laboratory. "These unique capabilities demonstrate the
flexibility and outstanding performance of this machine as we seek to
explore the subtle interplay of particles and forces that transformed the
QGP of the early universe into the matter that makes up our world today."

The nuclei of today's ordinary atoms and QGP represent two different
phases of matter whose constituents interact through the strongest of
Nature's forces. These interactions are described by a theory known as
quantum chromodynamics, or QCD, so scientists sometimes refer to the
exploration of QGP and this transition as the study of QCD matter.

As in other forms of matter, the different phases exist under different
conditions of temperature and density, which can be mapped out on a "phase
diagram," where the regions are separated by a phase boundary akin to
those that separate liquid water from ice and from steam. But in the case
of nuclear matter, scientists still are not sure where to draw those
boundary lines. RHIC is providing the first clues.

"RHIC is well positioned to explore QCD phase structure because we can
vary the collision energy over a wide range, and in so doing, change the
temperature and net quark density with which QCD matter is formed," said
Steven Vigdor, Brookhaven's Associate Laboratory Director for Nuclear and
Particle Physics, who leads the RHIC research program.

For example, physicists from RHIC's STAR and PHENIX collaborations have
analyzed results from gold ion collisions taking place at energies of 200
billion electron volts (GeV) per pair of colliding particles, all the way
down to 7.7 GeV.

While at the highest energies evidence for QGP formation is widely
accepted, "many of the signatures of the QGP developed at 200 GeV
disappear as the energy decreases," said STAR spokesperson Nu Xu, a
physicist at Lawrence Berkeley National Laboratory.

In particular, the STAR findings analyzed so far indicate that
interactions among "free" quarks and gluons-those characteristic of the
"perfect" liquid QGP discovered at RHIC-appear to dominate at energies
above 39 GeV, while at energies below 11.5 GeV, the interactions of bound
states of quarks and gluons known as hadrons (such as the protons and
neutrons of ordinary matter) appear to be the dominant feature observed.

The PHENIX experiment has observed similar behavior. They have found that
quarks passing through the matter produced at collision energies from 39
GeV upward lose energy rapidly, as anticipated for interactions within
QGP. Previous PHENIX results from copper-copper collisions at 22 GeV, in
contrast, are consistent with no significant energy loss.

These measurements are helping scientists plot definitive points, or
signposts, which tell them they may be approaching the boundary between
ordinary nuclear matter and the QGP that dominated the early universe. But
they haven't yet proven that a sharp boundary line exists, or found the
"critical endpoint" at the termination of that line.

"The critical endpoint, if it exists, occurs at a unique value of
temperature and density beyond which QGP and ordinary matter can
co-exist," said Vigdor. It is analogous to a critical point beyond which
liquid water and water vapor can co-exist in thermal equilibrium, he said.

Because of the complexity of QCD calculations, there is as yet no
consensus among theorists where the QCD critical point should lie or even
if it exists. But RHIC experimentalists say they see hints in the data
around 20 GeV that resemble signatures predicted to be observed near such
a QCD critical point. However, much more data from future experiment runs
at RHIC is required to turn these hints into conclusive evidence.

Apparent symmetry violations disappear at low energy

One signal that disappears in gold-gold collisions at RHIC energies below
11.5 GeV is the indication of a small separation of positive from negative
electric charge within the matter produced in each individual collision.
Ordinarily, such a charge separation would be forbidden by the "mirror
symmetry" that is a fundamental feature of QCD. But at the ultra-high
temperatures of QGP, the theory allows such symmetry violations to occur
in localized "bubbles," as long as they average out to zero when bubbles
from all collision events are looked at together.

"Such symmetry-violating bubbles are of crucial interest in the early,
high-temperature history of the universe, where analogous bubbles are
speculated to have played a central role in producing the preponderance of
matter over antimatter in today's universe, enabling our existence,"
Vigdor said.

The disappearing hints of charge separation may be another signal that the
lower-energy RHIC collisions are no longer producing QGP. But it's also
conceivable that the hints arise instead from a "background" phenomenon
that is related to the almond-like shape of the overlap region formed when
two spherical gold ions collide in not quite head-on fashion.

Head-on collisions of football-shaped uranium ions aligned in upright
positions like footballs set for kick-off-conducted for the first time
during the 2012 RHIC run, and made possible by a new ion source at
RHIC-are allowing scientists to study the effects of this almond-like
interaction region without the strong surrounding magnetic field also
produced in the off-center gold-gold collisions (which is necessary for
the interesting charge-separation signal).

Results so far, reported by STAR physicists at Quark Matter 2012, seem to
rule out the role of the background effect. If subsequent analysis
confirms this early finding, the uranium-uranium collisions will provide
further evidence for the symmetry-violating bubble interpretation of the
gold-gold data, and for the disappearance of QGP at the lower RHIC
energies.

From ordinary matter to plasma

The way quarks and gluons are arranged in ordinary matter affects how the
plasma forms, and also modifies production of experimental probes of the
plasma's properties. Teasing out effects of the plasma on these probes
requires good knowledge of the probes before they encounter QGP.

To get that important information, the RHIC experiments have collected a
large data set from collisions of gold ions with deuterons (the nuclei of
heavy hydrogen).

At Quark Matter 2012, PHENIX physicists report that there are fewer
high-momentum single hadrons and collections of hadrons called "jets"
produced in dead-on central deuteron-gold collisions than more glancing
deuteron-gold collisions.

"We expect jet suppression in quark-gluon plasma, because jets lose energy
in dense matter such as the plasma," said PHENIX spokesperson Barbara
Jacak, a physicist at Stony Brook University. "But this result shows that
we have to correct for this initial state effect when figuring out how
much the plasma suppresses the production of jets."

The initial state is related to the arrangement of quarks and gluons deep
inside the gold nucleus, which some theories predict could be a condensed
form of gluons called color-glass condensate, as hinted at in earlier
results published by PHENIX.

The force between quarks and antiquarks

Other new RHIC measurements reported at Quark Matter concern the
probability of heavy quarks (bottom and charm) and their anti-matter
counterparts pairing up to form bound states called "quarkonia" within the
QGP and in the "cold" nuclear matter probed in the deuteron-gold
collisions.

QCD tells us that the force between a quark and an antiquark increases in
strength as they are pulled apart, as though they were connected by an
invisible rubber band. But the strength of this force should be reduced in
QGP. So physicists expect the formation of quarkonia to also be reduced in
QGP, with the probability of finding such species decreasing with
larger-size bound states.

The STAR experiment reported new results consistent with this expectation
by studying different size bound states of bottom quarks and antiquarks.
PHENIX has studied suppression of bound states of charm and anti-charm
quarks in various beam combinations, both with and without plasma
formation. New results indicate that their formation is already suppressed
in collisions of deuterons with gold nuclei, when no QGP is formed.

"This reflects both the reduced production rates for heavy quarks and the
fact that the bound state sometimes breaks up as it passes through normal
(cold) nuclear matter," said Jacak. "It is crucial to quantify this if we
are to understand QGP effects on the binding," she said.

"These new results on the phase boundary, symmetry-violating bubbles,
initial state effects, and the production of quark-antiquark bound states
illustrate how scientists are exploiting RHIC's unique versatility for
precision determinations of the properties of quark-gluon plasma," Vigdor
said. "It is this versatility, in combination with dramatic advances we've
made in the rate of collisions provided at RHIC, that will allow our
scientists in the coming decade to answer the pointed questions raised by
RHIC's exciting discoveries about this early universe matter."

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